Welcome to Reef2Reef!!! In my opinion based on my experience, all of the contact spaces where the tank would normal touch the stand should be supported - not a just a shim at the corners or every 8” or so.
agree with @Peace River - can't just shim, you're putting too much weight and right next to shim may try to drop, which may eventually lead to seam failure and tank leaks
We needed to rip down 1x1s and create 'shims' that go entire length. . You can't see this in my tank photo, but there is a tiny strip of white entirely along floor under my tank. Room is a converted porch, so even with leveling compound making flatter, it's still not level. Where our stand supports come down, we cut 4x4s so grain goes up/down giving even more strength. If gravity were to fail and the stand and tank were to float up, you would find we mounted this "box (shim)" with painters tape (so it didn't slide on us when stand and tank went into room) which is fully level in all directions.
I shimmed the front of my 130 gallon stand every few inches because my basement floor slants to the front of the house, all old north east homes were built that way in order to funnel water to the front clean out in the floor in the event of flooding.